This invention relates to a semiconductor substrate, and in particular a mono-crystalline semiconductor plate-insulating film-mono-crystalline semiconductor film wherein the insulating film is formed with an opening and mono-crystalline semiconductor film is formed in the opening and on the insulating film.
There has been recent interest in growing single-crystal silicon films over insulating layers on silicon substrates as a substitute for Silicon On Sapphire substrates which are expensive. Additionally, methods for fabricating such devices can be utilized for fabrication of devices incorporating multiple isolated semiconductor layers. One such technique for preparing this type of substrate appeared in John C. C. Fan, M. W. Geis, and Bor-Yeu Tsaur, Appl. Phys. Lett. 38,(5), 365 (1981), wherein the authors have reported growing single-crystal silicon films over insulating layers on silicon substrates. According to this method, at the opening portion, or window, there is a step gap between the surface of the mono-crystalline semiconductive film on the semiconductive plate and a surface of the mono-crystalline semiconductor film on the insulating film. This same step gap is also formed in the mono-crystalline semiconductor film. Thus, a problem arises in that wiring formed at the window portion is easily damaged due to the step gap.
In these cases the conventional construction is not fully satisfactory in that parasitic capacitance which forms between the semiconductor device and the mono-crystalline semiconductor substrate cannot be reduced due to the necessarily thin insulating film. On the other hand, if the thickness of the insulating film is increased in order to reduce the parasitic capacitance effect, the step gap is larger. Such a large step gap tends to damage wires formed at the step gap.
The conventional manufacturing method of providing a monocrystalline semiconductor layer on an insulator is to provide a silicon semiconductor layer on a mono-crystalline sapphire substrate. This device is generally represented as SOS (Silicon On Sapphire) formed by the epitaxial method. However, in conventional techniques, there is no crystal surface wherein the lattice constant of the mono-crystalline sapphire substrate coincides with the lattice constant of the grown silicon mono-crystalline layer. The difference in lattice constant is within at least several percent at best. Thus, substantial crystal defects exist in the silicon mono-crystalline layer. Additionally, as such sapphire substrates are expensive, practical use of such substrates have been restrained. This is the case in spite of the increasing demand for manufacturing high-speed semiconductor devices, utilizes a mono-crystalline semiconductor layer on an insulator.
Accordingly, it would be desirable to provide a semiconductor substrate and a method of preparing the semiconductor substrate which would satisfy these needs and overcome the problems encountered in the prior art.